1. Field of the Invention
The invention relates to a control system.
2. Description of the Prior Art
The control system ensures that in a burner the ratio of the amount of air to the amount of fuel, called the air ratio or lambda, is matched in the whole performance range. As a rule lambda should be slightly above the stoichiometric value 1, for example, 1.3.
In a manner different to controlled burners, burners adjusted according to the air ratio react to external influences that change the combustion. For example, the combustion can be re-adjusted after a change in the type of fuel or the air density. It has a higher degree of effectiveness and thereby a greater efficiency and lower emissions of contaminants and soot. Environmental pollution is lower, and the lifespan is prolonged.
Burners adjusted according to the air ratio do not react properly to all the influences that change combustion, however, regardless of their construction. Adjustment can thus sooner or later become inaccurate. This makes regular monitoring or calibration advantageous.
Typically, calibration is required because the air resistance in the air supply duct or exhaust duct increases due to emission of soot or to foreign bodies. Rather less often, but equally affecting, is the reverse case, where the air resistance decreases because of unintentional perforation of the ducts. Often, the inflow of air is determined by an external performance signal, and the control system adjusts the inflow of fuel to a reference control value corresponding thereto, even if the actual inflow of air no longer conforms to the performance signal.
Many a large burner senses the airflow volume, for example, via pressure sensors, and can thus compensate for changes to it to a certain extent. Sooner or later the sensor result will no longer be reliable, however. Simple burners rely exclusively on the air blower speed generated by a Hall sensor or on the position of a flap resulting from electrical resistance measurement, or even just on the present actuating signal.
DE-A1-4429157 discloses a monitoring method for air ratio control. Changing of a second parameter as a result of the change in a first parameter is observed, that is to say the changed value of the sensor signal in response to a controlled, fixed change in an actuator. It is decided, from the difference between this observation and stored reference values, whether a fault indication will be output.
DE-A1-4429157 did not specify for which types of sensor the monitoring method is suitable. The same applicant later described a completely different calibration method specifically for burners in which the sensor for controlling the air ratio is formed by the ionisation electrode. This is found in EP-A2-1002997, and will be described hereinafter.
Control of the air ratio is particularly effective when the combustion quality can be directly or indirectly observed with a sensor. Typically with known burners, oxygen sensors are used in the exhaust gas duct, temperature sensors on the burner surface, or UV sensors in the combustion chamber. These sensors are expensive, unreliable and high-maintenance for this purpose, and/or have a short lifespan.
Newer developments are based on the ionisation electrode that has been used as standard for a long time for monitoring the flame in burners. Although it is not easy to evaluate its signal, the ionisation electrode does not have the disadvantages described hereinabove.
Any changes to the ionisation electrode typically require calibration. It can be changed by bending, wear or chemical attack to its surface, or by soiling with soot particles. The control system then attempts to structure the combustion erroneously, such that measurements at a reference control value based on an unchanged ionisation electrode are kept to.
According to EP-B1-770824, in order to calibrate a burner controlled by ionisation flow where there is a fixed fuel flow volume, the air supply is reduced from its controlled value, past a point where the ionisation signal reaches its maximum stoichiometric value. This maximum is established. The difference between the newly established and the stored, previous maximum, allows the burner control to determine new reference ionisation values for the combustion at the desired air ratio.
By means of richer combustion, this method enables simple and reliable determination of a measured value without playing a role in at what level of air supply this measured value is obtained or how the ionisation signal depends exactly on the air supply.
Also in EP-A2-1002997 and DE-C1-19854824, calibration methods for ionisation flow controlled burners have been described. These include sensing of a second measured signal that is representative of the present performance, even though in connection with the ionisation signal. The second measured signal is always a second, different type of ionisation signal in DE-C1-19854824 and in particular instances in EP-A2-1002997.
In DE-C1-19854824, it is apparently assumed that the second ionisation signal is barely sensitive enough for the instantaneous values of the burner performance in terms of measurement of the thermal electron output from the ionisation electrode. Similarly, according to EP-A2-1002997 a second ionisation signal can be generated that, in contrast to the first, is dependent upon the burner performance and the air ratio, and in fact with the aid of a special evaluation circuit is absolutely no longer dependent upon the burner performance.
The known calibration methods do not at present change the normal operation of the burner. Testing is simply done as to whether the burner performance and the air ratio from the different ionisation signals are still in agreement with one another. When this is the case, the control method remains unchanged. Only when certain threshold values have been exceeded is it adapted.
Controlling adaptation takes place in DE-C1-19854824 in that the inflow determined from the performance signal, for example, the inflow of air, is changed until the second ionisation signal has an acceptable value again. Meanwhile, the other inflow is readjusted exactly as previously. In this way erroneous changes to the air ratio and to the burner performance should be reversed. As soon as a stable state is obtained, the first ionisation signal is measured. Lastly, this measured value is adopted as the new reference value for the first ionisation signal.
Provided that the second measured signal is reliable, this method can test whether the correct air ratio is present, without affecting controlling. The changes required for any controlling adaptation are immediately effective as they are simply to be directed towards approximation to the fixed value of the second measured signal or to the desired relationship thereof to the ionisation signal.